Bartok's Mikrokosmos are pretty much an introduction to the new styles of the twentieth century (from whole tone scales to atonality to tone clusters...). They go in ascending difficulty (as in vol. 1 is much easier than the later ones). As to where to find them on the internet, I don't know where they would be free, but you should be able to find them on any reputable sheet music distributor.
Ex.- the website Sheet Music Plus